The present invention relates to a method and device for forming porous ceramic coatings on metal substrates, in particular thermal barrier coatings on gas turbine components; to ceramic coatings formed by such a method; and to metal components applied with such coatings.
As is known, to increase the operating temperature of gas turbine combustion chambers for the purpose of improving efficiency of the turbine and reducing pollutant emissions (particularly nitric oxide), the turbine components subjected to critical thermal and oxidation conditions are made of special high-resistance materials, such as nickel-based superalloys, and are protected by ceramic or so-called thermal barrier (TBC) coatings typically formed by plasma spraying, which consists in spraying ceramic powder on to the workpiece by means of a plasma gas jet.
Despite the already high performance of known thermal barrier coatings, particularly in terms of gas combustion temperature and component life and reliability, further improvement in insulation capacity is hoped for to enable an even greater increase in efficiency of the turbine and a further reduction in pollutant emissions.
The efficiency of thermal barrier coatings in ensuring maximum thermal insulation is also known to increase in proportion to the porosity of the ceramic deposit. Thermal barrier coatings with a porous structure therefore provide for better insulation as compared with compact coatings, but involve complex adjustments in optimum ceramic deposition parameters to achieve good mechanical properties and high deposition efficiency (defined as the adhesion probability of the sprayed particles, i.e. the ratio between the material actually deposited and the powder supplied to the plasma torch). As a result, porous thermal barrier coatings are generally characterized by low deposition efficiency (and hence high consumption of ceramic material) and poor mechanical performance.
Finally, known thermal barrier coatings are normally of limited thickness--less than 1 mm--due to the tendency of thicker ceramic coatings to become detached as a result of the rapid variations in temperature to which the components are subjected.